Rats Laugh When You Tickle Them

It may sound surprising, but the roots of joyful play run deep in the mammalian brain. While we can't hear it with our own ears, rats emit a distinct, high-frequency chirp when they're happy. This discovery was made by the pioneering neuroscientist Jaak Panksepp, who was studying the neurological basis of emotion. He found that when he playfully tickled young rats, particularly on their bellies and the napes of their necks, his recording equipment picked up a flood of 50-kilohertz ultrasonic vocalizations. Panksepp identified these sounds as a fundamental expression of positive social feeling, an ancient form of laughter.

The evidence for their enjoyment goes beyond just sound. The rats that were tickled became socially bonded to the researchers, actively seeking out and chasing their hands to solicit more play. They would even perform what German ethologists call "Freudensprünge," or "joy jumps," during these playful encounters. Panksepp's groundbreaking work demonstrated that the capacity for joy and the instinct for play are not uniquely human traits but are deeply embedded in the brain structures we share with many other animals, providing a powerful window into the evolution of our own emotions.